Plastics are used in an increasing number of applications, such as for electronic devices, miscellaneous goods, automotive components, and so forth, due to their light weight compared to metals and the like. There is demand for plastic components having excellent physical properties in terms of weight-reduction performance, strength, impact resistance, and so forth in consideration of energy efficiency and the like. One known technique for responding to this demand is the technique of producing resin foam shaped products.
Examples of conventionally used plastics include general-purpose resins such as polyethylene, polystyrene, and polypropylene that excel in terms of weight-reduction performance, impact resistance, and so forth. Foam shaped products of these general-purpose resins are suitably used as cushioning materials and the like.
However, due to the poor heat resistance of these general-purpose resins, it has not been possible to use foam shaped products of these general-purpose resins for components that are required to have high heat resistance, such as automotive components.
On the other hand, engineering resins, and particularly polyamide resins, are known as plastics having high heat resistance in addition to excellent wear resistance, chemical resistance, and so forth.
Accordingly, foam shaped products of polyamide resins are considered to be of use in applications for which higher heat resistance is required.
Examples of techniques for producing resin foam shaped products include extrusion foaming, foam injection molding, and in-mold foam shaping (also referred to as bead foam shaping).
Extrusion foaming is a method in which an organic or inorganic foaming agent is injected into a molten resin using an extruder and the pressure is subsequently released at an outlet of the extruder to obtain plate, sheet, or columnar foam having a specific cross-sectional shape, and then the foam is heat processed in a mold or is cut and pasted to shape the foam into a desired shape.
Foam injection molding is a method in which an expandable resin is injection molded and foamed in a mold to obtain a foam molded product including pores.
In-mold foam shaping is a method in which expandable resin particles are loaded into a mold, are heated by steam or the like, and are foamed and simultaneously thermally fused to one another to obtain a foam molded product. In-mold foam shaping is widely used in industry due to advantages such as ease of freely setting the product shape, ease of obtaining a foam molded product with a high expansion ratio, and so forth.
Up until the present, there has been no example of production of a polyamide resin foam shaped product by bead foam shaping and no example in which it has been speculated that industrial production of such a polyamide resin foam shaped product is possible.
PTL 1, for example, discloses a technique in which polyamide resin particles and methyl alcohol are supplied into an autoclave with water solvent, the system is heated and subsequently exposed to atmospheric pressure to obtain pre-expanded particles, and then these pre-expanded particles are loaded into a mold of a shaping machine and are shaped to obtain a polyamide resin foam shaped product.
PTL 2 discloses a technique in which carbon dioxide gas is blown into polyamide resin particles in an autoclave, the resultant particles are heated to obtain pre-expanded particles, and then these pre-expanded particles are loaded into a mold of a shaping machine and are heated by hot air to obtain a polyamide resin foam product.